Bumpers typically extend widthwise across the front and rear of a vehicle and are mounted to rails which extend in a lengthwise direction. Desirable bumper energy absorbing systems minimize vehicle damage by managing the impact energy of a collision with a minimal amount of intrusion while not exceeding the rail load limit of the vehicle. The ideal energy absorber achieves high efficiency by building load quickly to just under the rail load limit and maintains that load constant until the impact energy has been dissipated.
A foam type bumper is described in U.S. Pat. No. 4,762,352 to Kaisha. A backup beam is connected to a pair of rails. A foam type resin of polypropylene, polyurethane or the like is positioned between the backup beam and an outer fascia.
Another system of the foam type is described in U.S. Pat. No. 4,941,701 to Loren. As set forth, a semi-rigid resilient fascia is spaced forwardly of the bumper structure, and the volume defined therebetween is filled with an integral skin urethane foam that is resiliently deformable and integrally bonded to both members.
Disadvantages of foam type systems include slow loading upon impact which results in a high displacement. Typically, foams are effective to a sixty to seventy percent compression. Beyond this point, foams become incompressible so that the impact energy is not fully absorbed. The remaining impact energy is absorbed through deformation of the backup beam and/or vehicle structure. Foams are also temperature sensitive so that displacement and impact absorption behavior can change substantially with temperature. Typically, as temperature is lowered, foam becomes more rigid resulting in higher loads. Conversely, as temperature rises, foams become more compliant resulting in higher displacements and possible vehicle damage.
U.S. Pat. No. 4,533,166 to Stokes describes non-foam energy absorbing system using a channel section shaped inner beam positioned inside a contoured outer beam having a channel shape cross section. The outer beam has transverse ribs stiffening the vertical portion with longitudinal stiffeners running between the ribs. The inner beam has transverse interbeam support sections molded in the exterior sides of the beam. Interbeam supports are positioned longitudinally displaced from the bumper supports to affixes the outer beam member to the inner beam member in spaced apart relationship. The double beam bumper is designed to be relatively insensitive to the location of the point of impact as far as energy absorption and impact forces are concerned with the purpose of eliminating bumper shock absorbers. The system requires separately molded outer beam and inner beam members having a particular shape.
The present invention is directed to an energy absorbing system of the non-foam type which is designed to achieve fast loading and efficient energy absorption upon impact. Impact forces during low speed impacts are maintained just below a predetermined level by deforming the energy absorber until the kinetic energy of the impact event has been absorbed. Once the impact is over, the absorber returns substantially to its original shape and retains sufficient integrity to withstand subsequent impacts.